Extract flickr_mail package with Mapped models and shared utilities

Move from JSON file storage to SQLite database using SQLAlchemy with
Mapped type hints. Deduplicate URL utility functions into shared
flickr_mail package.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flickr_mail/database.py

@@ -0,0 +1,31 @@
+"""Database engine and session factory for flickr-mail."""
+
+from pathlib import Path
+
+from sqlalchemy import create_engine, event
+from sqlalchemy.orm import Session, sessionmaker
+
+from flickr_mail.models import Base
+
+DB_PATH = Path(__file__).parent.parent / "flickr_mail.db"
+
+engine = create_engine(f"sqlite:///{DB_PATH}")
+SessionLocal = sessionmaker(bind=engine)
+
+
+@event.listens_for(engine, "connect")
+def set_sqlite_pragma(dbapi_connection, connection_record):
+    """Enable WAL mode for concurrent read/write access."""
+    cursor = dbapi_connection.cursor()
+    cursor.execute("PRAGMA journal_mode=WAL")
+    cursor.close()
+
+
+def init_db() -> None:
+    """Create all tables."""
+    Base.metadata.create_all(engine)
+
+
+def get_session() -> Session:
+    """Create a new database session."""
+    return SessionLocal()